An ion trap is a device used in a mass spectrometer to capture and confine ions by an action of a radio-frequency electric field, to select an ion having a specific mass-to-charge ratio (m/z), and to dissociate the selected ion into fragment ions. A typical ion trap has a three-dimensional quadrupole structure including a circular ring electrode and a pair of end-cap electrodes facing each other across the ring electrode, where the inner surface of the ring electrode is in the form of a hyperboloid of revolution of one sheet while the inner surfaces of the end-cap electrodes are in the form of a hyperboloid of revolution of two sheets. Another commonly known type of ion trap is a linear type having four rod electrodes arranged parallel to each other. In the present description, the “three-dimensional quadrupole type” is taken as an example for convenience.
A majority of conventional ion traps are analogue-driven ion traps, which will be described later. In an analogue-driven ion trap, a sinusoidal radio-frequency voltage is normally applied to the ring electrode to create an ion-capturing radio-frequency electric field within the space surrounded by the ring electrode and the end-cap electrodes. Due to the action of this radio-frequency electric field, ions are confined in the aforementioned space while oscillating in this space. In recent years, a new type of ion trap, in which a rectangular-wave voltage is applied to the ring electrode in place of the sinusoidal radio-frequency voltage to confine ions, has been developed (for example, see Patent Documents 1, 2 or Non-Patent Document 1). This ion trap normally uses a rectangular-wave voltage having the binary voltage levels of “high” and “low” and is therefore called a Digital Ion Trap (which is hereinafter abbreviated as “DIT”).
In an MS/MS analysis performed by an ion trap mass spectrometer using a DIT (which is hereinafter referred to as the “DIT-MS”), after ions within a predetermined mass-to-charge ratio range have been captured into the inner space of the ion trap, a precursor-isolating (selecting) operation for ejecting unnecessary ions from the ion trap must be performed to leave only an ion having a specific mass-to-charge ratio. As described in Non-Patent Document 1, the techniques of precursor isolation in the DIT-MS include high-speed precursor isolation, which is called rough isolation, and high-resolution precursor isolation, which is performed using resonant ejection after the rough isolation.
One advantage of the DIT over the analogue-driven ion trap (hereinafter abbreviated as the “AIT”) using a sinusoidal radio-frequency voltage is the high mass-resolving power achieved by resonant ejection. Normally, in the resonant ejection performed in a DIT, a rectangular-wave signal having a single frequency synchronized with the frequency Ω of the rectangular-wave voltage applied to the ring electrode is applied to the pair of the end-cap electrodes, where the aforementioned single frequency is typically obtained by dividing the aforementioned rectangular-wave voltage. In this state, when the frequency Ω of the rectangular-wave voltage applied to the ring electrode is continuously decreased, the ions captured in the ion trap are selectively subjected to resonant excitation in ascending order of their mass-to-charge ratio and ejected from the ion trap. (This operation is called a “forward scan”, where an ion having a smaller mass-to-charge ratio is ejected earlier.) Conversely, when the frequency Ω of the rectangular-wave voltage applied to the ring electrode is continuously increased, the ions captured in the ion trap are selectively subjected to resonant excitation in descending order of their mass-to-charge ratio and ejected from the ion trap. (This operation is called a “reverse scan”, where an ion having a larger mass-to-charge ratio is ejected earlier.) Accordingly, it is possible to achieve a high level of precursor-isolation power by successively performing the forward scan and the reverse scan so as to leave only an ion having a desired mass-to-charge ratio.
In order to completely remove ions having unnecessary mass-to-charge ratios, it is necessary to hold the frequency Ω for a period of time required to eject those ions from the ion trap. For this purpose, the speed of changing the frequency Ω must be set lower than a certain speed. Therefore, to achieve a sufficient mass-separating power, a period of time equal to or longer than several hundred msec is required for only the precursor isolation. For example, in the case of an MS/MS analysis including the steps of (A) trapping ions within a predetermined mass-to-charge ratio range in the ion trap and cooling them, (B) removing undesired ions by resonant ejection to retain only a precursor ion (the precursor-isolating step), (C) inducing collision dissociation of the precursor ion, and (D) extracting the collision-dissociated ions by resonant ejection and obtaining a mass spectrum, each of the steps (A), (C) and (D) requires a few to several tens of msec. Consuming several hundreds of msec for only step (B) will significantly lower the throughput of the analysis. In recent years, improving the throughput of the mass analysis has been extremely important. Therefore, time reduction of the precursor isolation in the DIT is a critical and unavoidable problem.
In the previously described ion-removing method using a frequency scan, a portion of the ions that are resonantly excited to be ejected from the ion trap may undergo collision-induced dissociation, generating fragment ions having smaller mass-to-charge ratios. Furthermore, although this is a rare case, a multiply-charged ion may turn into an ion having a larger mass-to-charge ratio as a result of charge transfer and dissociation. After unnecessary ions have been removed by a frequency scan to leave an ion with a certain mass-to-charge ratio, if fragment ions or other ions are generated as just described, these ions cannot be removed by the frequency scan and will remain inside the ion trap.
By the way, in the case of AITs, the oscillation frequency of the ions changes depending on the amplitude of the radio-frequency voltage applied to the ring electrode. Based on this relationship, a technique for simultaneously removing various kinds of ions having unnecessary mass-to-charge ratios other than the target ion (precursor ion) has been developed, in which a signal having a broad-band frequency spectrum with a notch (omission) at the oscillation frequency of the target ion is applied to the end-cap electrodes (for example, see Patent Document 3 or 4). One example of the signals commonly used as the aforementioned broad-band signal is a Filtered Noise Field (FNF) signal described in Patent Document 5. Another conventional example is a Stored Wave Inverse Fourier Transform (SWIFT) signal described in Patent Document 6.
To achieve a high level of mass-separating power, it is necessary to perform the precursor isolation with the highest possible q value, which is one of the parameters representing the conditions for the stable capturing of ions. For AITs, the q value is normally set at approximately 0.8. When the q value is fixed, the β value (a parameter associated with the resonance frequency) will also be fixed, whereby the notch frequency of the FNF signal will be uniquely determined. By preparing a dozen or more FNF signal waveforms having different notch widths centered on the notch frequency and storing them in a memory beforehand, it is possible to easily achieve precursor isolation with a given mass width by selecting an appropriate FNF signal waveform for the precursor isolation.
The idea of isolating a precursor ion by using an FNF signal or similar broad-band signal is also applicable to the DIT as well as the MT. It should be noted that, unlike the case of the AIT, the amplitude of the rectangular-wave radio-frequency voltage applied to the ring electrode in the DIT is basically constant; it is the frequency of the rectangular-wave voltage that is changed to control the oscillation frequency of the ions. Accordingly, for the DIT, it is possible to adopt a method in which the notch frequency of the FNF signal applied to the end-cap electrodes is fixed and the frequency of the rectangular-wave voltage applied to the ring electrode is controlled so as to make the oscillation frequency of the target ion correspond to the notch frequency. However, such a control causes the q value used in the precursor-isolating process to change according to the mass-to-charge ratio of the target ion. This is because, as will be described later, the q value is a function inversely proportional to the square of the frequency of the rectangular-wave voltage applied to the ring electrode. Therefore, it is impossible to ensure a sufficiently high mass-separating power under the condition where the q value is decreased.
To avoid such a situation, the q value must be maintained as constant as possible during the precursor-isolating process. For this purpose, when the mass-to-charge ratio of the target ion is changed, it is necessary to change not only the frequency of the rectangular-wave voltage applied to the ring electrode, but also the notch frequency of the FNF signal supplied to the end-cap electrodes in accordance with the change in the frequency of the rectangular-wave voltage. Generating an FNF signal having a large number of frequency components by a computer normally requires a considerable period of time, and it is impractical to generate a required FNF signal waveform on a computer simultaneously while performing an analysis. Therefore, in the case of using an FNF signal in an AIT, an FNF signal waveform that is expected to be required is generated beforehand on a computer and a data representing the waveform is stored in a memory. When an analysis is performed, the data is read from the memory and subjected to digital-to-analogue conversion to produce the FNF signal waveform.
In the case of DITs, as already explained, various FNF signal waveforms with different notch frequencies are required. Therefore, it is necessary to prepare a large number of FNF signal waveform data corresponding to those waveforms and store the data in a memory. For example, to make the notch frequency selectable within a mass-to-charge ratio range from m/z50 to m/z3000 in units of 0.1, it is necessary to prepare approximately 30,000 kinds of different FNF signal waveform data. Furthermore, to enable the selection of various mass-separation widths, it is further necessary to prepare several tens of different waveforms for each value of the notch frequency. As a result, the amount of required FNF signal waveform data will be an enormous number.